(1) Field of the Invention
This invention relates to a drying apparatus which is used to dry and season lumber.
(2) Description of the Prior Art
Most artificially dried lumber is dried by the conventional batch kiln process, whereby the moisture content of the boards is carefully reduced to a desired level to produce the seasoned lumber. The kiln-dried lumber has traditionally been dried below 200.degree. F with relative humidities kept high (40% to 80%) to avoid degradation of the wood, but more recently equipment has been developed to dry wood at temperatures above 212.degree. F (high temperature drying). The limits of temperatures in commercial high temperature dryers have been about 250.degree. F for hardwoods and 270.degree. F for softwoods. Although high temperature drying reduces drying time compared to conventional kiln drying, high-temperature dried wood is more susceptible during drying to warp, collapse, end-checking, honeycomb, and casehardening than low-temperature dried wood.
Lumber dryers in the past have used circulation of air parallel to the drying surface. Impingement drying, whereby the gases are ejected perpendicular to the surface of the drying medium through slots or holes to form small air jets, has been commercially used for veneer drying. The impingement of the air increases the energy transfer to the veneer surface when compared to parallel air flow and thus increases drying efficiency.
To effectively use the impinging action of the air jets in an air impinging dryer, the air openings or orifice plates must be at a proper distance away from the surface of the wood, usually less than 2 inches. The usual impingement dryer is used to dry thin material of less than 1/8 inch and thus there is no need for adjusting the space between opposing banks of air jets. Lumber, ranging in thickness from 1/2 inch to 4 inches, requires adjustment of the opposing banks of air jets to effectively dry the wood.
Since the usual impingement dryer is used to dry thin material, drying times are very short compared to the time to dry lumber. A minimum lumber conveyance rate on the order of 1 foot per minute is required to maintain even drying and to prevent scorching of the wood surface by the air jets at higher drying temperatures. Drying of wood even at the most severe drying conditions takes several hours; thus, lumber would have to be passed several times through the usual continuous dryer in order to maintain the proper movement of the lumber.
Lumber drying can be divided into three stages: from initial moisture content to the fiber saturation point throughout the wood, from the surface first falling below the fiber saturation point to the wood falling below the fiber saturation point throughout, and from the wood falling below the fiber saturation point throughout to the final moisture content. The air velocity has a diminishing effect on drying rate in each of these stages from initial to final moisture content.
High quality lumber requires a minimum of physical defects in the dried lumber. Conventional kilns use a high humidity stress relief cycle at the end of the drying to relieve casehardening. Warpage is reduced in conventional kilns by applying heavy weights at the top of the kiln stack and in continuous dryers by compression rollers. The rollers in continuous kilns tend to gum with exudates from the wood and extra mechanical energy is required to force the wood against the restraining action of the rollers.
The main objective of this invention is to provide a more efficient and effective dryer for continuously drying lumber.
An object of this invention is to provide a continuous wood dryer with steam seasoning capabilities.
Another object of this invention is to provide a separate steaming section, whereby wood traveling through this section is relieved of internal stresses by the impingement of steam.
Another object is to provide a dryer that will accept a wide range of lumber thicknesses and still maintain the close distances between the lumber and ejector jets.
Another object of this invention is to provide a means by which lumber can be conveyed through a dryer of reasonable size without scorching the lumber or requiring multiple passes through the dryer for long drying times.
Another object of this invention is to provide a continuous dryer with a conveyance mechanism capable of handling a wide range of lumber thicknesses while mechanically restraining the wood to prevent excessive warpage.
As discussed earlier in this disclosure, higher air velocities are more efficiently used in the first stage of drying. Another object of this invention is to provide variable air flow control along the length of the dryer to more efficiently use the air in drying the wood.